Weaning conflict, common to many mammalian species, is a phenomenon that arises when both mother and suckling offspring disagree over further continuation of maternal investment. This conflict generally involves repeated rejection of offspring by the mother, which may be countered by the offspring with two weaning delay strategies: 1) regression, or periodic recurrences of more infantile-like behavior which enable the young to approach and suckle, and/or 2) thief suckling, or milk stealing, from unrelated females. Both strategies are adaptive in that they permit offspring to garner extra investment beyond that which their mothers care to provide. Robert Trivers hypothesized that one factor influencing the duration and intensity of weaning conflict between mother and offspring is the mother's future potential of producing more offspring, termed 'reproductive value', which is an inverse function of her age. This hypothesis remains untested, however, because its substantiation requires accurate age and genealogical data of study subjects. Likewise, regression and thief suckling have not been investigated in relation to weaning conflict. The purpose of the proposed study is to test one of Trivers' weaning conflict hypotheses and to study the roles of regression and thief suckling as weaning delay strategies. A herd of scimitar-horned oryx (Oryx dammah), situated under semi-natural conditions at the National Zoo's Conservationa and Research Center, Front Royal, Virginia, is ideally suited for this study because accurate age and genealogical data on each herd member are available, the herd can be closely monitored within the 25 acre pasture, and other ungulate studies are in progress at the Center that are designed to provide comparable data. Specific hypotheses to be tested are: 1) Weaning conflict should be more intense between younger mothers and their offspring than between older mothers and their young, 2) Regressive behavior should be more prevalent in offspring with younger mothers, 3) Sexual differences in the occurrences of regression and thief suckling should exist between calves, and 4) Thief suckling should be directed to unrelated females.